Lower Palceozoic Rocks of the South- East of Ireland. 595 



From the preceding Table we find the following mineralogical results: — 



Table IV. 



Atomic proportions of White Mica. Atoms 



Silica, .... 

 iUumina, . . . 

 Peroxide of iron, 

 Lime, .... 

 Magnesia, . . 

 Potasb, . . . 

 Soda, .... 

 Protoxide of iron. 

 Water, . . . 



990 

 618 Ifi-, 



56/*'^-' 



28^ 



38 

 227 



30 

 2J 

 593 



^325 



In the preceding analyses I only determined the quantity of protoxide of 

 ii'on in the fourth ; but as there is probably a small quantity in the other 

 three, there should be something deducted from the atomic weight of the perox- 

 ides, and something added to that of the protoxides; we, therefore, obtain very 

 accurately the following relation for the atomic proportions of silica, peroxides, 

 and protoxides : — 



Silica 99, . . 3 



Peroxides, ... 66, .. 2 

 Protoxides, ... 33, .. 1 

 Water, . . . . 60, . . 2 ? 



giving a mineralogical formula for the white mica — 



RO, SiOj + 2 [RjOa, SiOs] + 2H0. ( 3 ) 



This is the mica known to mineralogists as Margarodite or pearl mica, and 

 improperly considered by Dana as an altered Muscovite. It is a distinct 

 mineral species, and one highly interesting in connexion with the theory of 

 the origin of granite, as it is undoubtedly a hydrated mineral, not the result of 

 metamorphic action, but coeval with the other minerals forming the mass of 

 the granite. 



If its formula be deduced from its oxygen, and not from its atomic i)ropor- 

 tions, we obtain from the mean of my four analyses — 



0-14 (3R0) + 0-86 (RA) + 1-26 (SiOs), 



